Shallow trench isolation (STI) is a common isolation technology for insulating active areas in integrated circuits, particularly integrated circuits with sub-quarter micron dimensions. An example of one common shallow trench isolation structure is shown in FIG. 1, wherein a wafer 100 has a substrate 110 having isolation trenches 112 formed therein. The substrate 110 is typically silicon. The isolation trenches 112 are typically filled with a dielectric material, such as a silicon oxide or another oxide. The isolation trenches 112 isolate active regions 116 from each other.
During fabrication, oxidation of the sidewalls of the isolation trenches may occur during subsequent processing steps. As a result of the oxidation and the volume expansion that occurs during the oxidation, however, compressive stress may be induced in the adjacent active regions 116. The compressive stress results because of volume expansion in a confined space.
To solve this problem, several approaches have been proposed for the formation of trench isolation structures with nitride liners. Generally, these approaches use a nitride liner to prohibit or reduce further trench sidewall oxidation in order to reduce the stress introduced by the trench filling material. For example, U.S. Pat. No. 5,447,884 issued to Fahey et al. describes a shallow isolation trench with a thin nitride liner; U.S. Pat. No. 6,461,937 issued to Kim et al. describes methods of forming trench isolation structures containing a silicon nitride layer stress relief layer; U.S. Pat. No. 6,251,746 issued to Hong et al., describes methods of forming trench isolation regions with stress-reducing nitride layers; and U.S. Pat. Nos. 6,461,937 and 6,251,746 describe nitride layers overlying a silicon oxide layer that is typically thermally grown at an elevated temperature.
While the nitride liners prohibit further oxidation of the trench sidewalls and, thus, reduce the compressive stress that results, nitride liners are usually formed with an intrinsic stress in the film. In addition, due to the intrinsic stress in the nitride liner, defects or even cracks may be formed at weak regions in the active regions, such as at sharp corners.
Furthermore, approaches have been proposed to use an oxide liner beneath the nitride liner, thus reducing the influence of the nitride liner on the active region. The use of an oxide liner, however, typically involves a high thermal budget.
Therefore, there is a need for a method of forming isolation trenches to prohibit or reduce the effect of oxidation of the isolation trench sidewalls.